This invention relates generally to refrigeration systems and in particular, to a method for preventing overheating of the compressor of a refrigeration system. The invention is particularly well-suited for converting an existing refrigeration system using one refrigerant having particular physical and thermodynamic properties to use with another refrigerant having significantly different properties.
The design specifications of a refrigeration system are generally predicated on the choice of specific refrigerant to be utilized, i.e., on its physical and thermodynamic properties. For years, chlorofluorocarbons, e.g., CFC-12 or R-12; CFC-502 or R-502, had been used in compression refrigeration systems. These chlorofluorocarbons have excellent stability and were well suited for low temperature applications.
During the past two decades, it has been found that such chlorofluorocarbons released into the earth's atmosphere were depleting the ozone layer. Reduction in the ozone layer has been linked to many effects such as an increased risk for skin cancer. In response to concerns over ozone layer depletion, the U.S. government has imposed increasingly stricter limitations on the use of these refrigerants. These limitations require the phase out of the commonly used refrigerants with other refrigerants considered not so effect the ozone layer.
Currently, many commercial refrigeration systems utilize R-502 and the design features of such systems are dictated by the properties of R-502, e.g., type, size and operating parameters of the compressor. The phase out of R-502 in favor of other refrigerants, such as R-22 or AZ-50, is not a simple matter of removing the refrigerant from the existing system and replacing it with the environmentally preferred refrigerant. The physical and thermodynamic properties of, e.g., R-22, refrigerant are significantly different from those of R-502 such that the refrigeration system operates with different performance parameters than those required by R-502.
In the normal compression refrigeration cycle, vapor refrigerant is drawn into a compressor where it is compressed to a higher pressure. The compressed vapor refrigerant is cooled and condensed in a condenser into a high pressure liquid which is then expanded, typically through an expansion valve, to a lower pressure and caused to evaporate in an evaporator to thereby draw heat and thus, provide the desired cooling effect. The expanded, relatively low pressure vapor refrigerant exiting the evaporator is once again drawn into the compressor and the cycle starts anew.
The action of compressing the vapor refrigerant imparts work onto the vapor and results in a significant increase in the vapor temperature. While a substantial portion of this heat is subsequently transferred to the atmosphere during the condensation process, a portion of the heat is transferred to the compressor components. Depending upon the specific refrigerant vapor compressed and on the pressure conditions operation, this heat transfer can cause the temperature of the compressor components to overheat, resulting in degradation of compressor performance, of the compressor lubricant or oil, and potentially damage to the compressor itself. For example, it has been found that the direct substitution of R-22 for R-502 in an existing refrigeration system results in high discharge temperatures, particularly under high load situations and high compression ratios.
One solution for converting existing systems using R-502 to R-22 or other substitutes calls for the replacement of expensive equipment, e.g., the compressor or supplementation of the existing condenser, resulting in significant capital costs as well as higher operating costs due to increase capacity needed for the compressor and condenser. Some prior art systems have attempted to respond to this problem. See, e.g., U.S. Pat. No. 5,189,883 issued to Bradford which discloses a refrigeration retrofit system utilizing a liquid refrigerant injection system, and U.S. Pat. Nos. 5,076,067 issued to Prenger et al. and 4,974,427 issued to Diab which also disclose a liquid refrigerant injection system for limiting or controlling excessive discharge gas temperature. See, also, U.S. Pat. No. 5,329,788 issued to Cailliat et al., and U.S. Pat. No. 5,640,854 issued to Fogt et al. These prior art systems, however, require the installation of multiple components to an existing system, require significant structural modification to an existing system or do not permit at all modification to an existing system.
Despite recognition and study of various aspects of the replacement refrigerant problem, the prior art has still not produced a simple, economical way to convert existing compression type commercial and industrial systems designed, e.g., for R-502, to the use of newer, environmentally preferred refrigerants.